Yamha 2002 225 four stroke rough idle, then misfires

[Amy]

I have a yamaha 225 four stroke that has been running good except for a bit of roughness when brought back to idle after running at crusing speed. Today it ran fine at crusing speed, idled for about 10-15 minutes then cruised back to inlet and motor seemed like it was misfiring? Even stoped and lifted it up as it felt like prop had hit something but there was nothing wrong. Motor then picked up and ran fine until we entered inlet came back to idle of a few minutes and the hit the gas again. Same misfires then engine shut off. I have twins so returned to dock with one engine. In the slip the one misfiring engine would start but shut off after about 10 seconds. Went home and got my computer with diaognastic progam and ran tests everything says normal. Even ran engine for about 15 minutes at dock, no roughness no misfires. Anyone have any idea?

Any help would be apperciated

[boats.net]

First thing that comes to mind is water in fuel, have you check fuel filters? You might try a new set of spark plugs. The next thing would be fuel pressure and vacumn test.